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Mixamo
There is none. That`s why you still need an artist.
There are tools like Makehuman with which you can shape humans. But it is still not a one click solution to have your game ready rigged character.
Mixamo is just another shop where you buy your game ready models. Plus it provides a somehow fast method to rig and animate them then by motion capturing files.
quadium
You need to make your own or pay someone to make your own models, pretty much.
While asset store items are good and that, in the end, you need to have something unique to your game, and that means that even with the store bought characters you need to change them so much that they are unique.
There is no solution that will make original characters for you. Things like Daz 3d, Make Human, and Poser are in most cases inadequate for gaming. They’re ridiculously high poly, which means… you’ll have to retopologize the mesh and use just high definition textures. But that means you’re back to creating stuff the hard way.
So… my advice is, Blender. Blender and a cube.
Unless you buy a model, it’s almost impossible.
Obviously, Solid Snake wasn’t made in a few minutes, nor was Ironman.
Did you mean Quidam? http://www.n-sided.com/
There is also good ol’ POSER.
There were few games that were made with Poser characters (as far as I know) - majority of them Indie point-and-click ADVENTURE GAMES (which are mostly 2D rendered sprites). Poser character models are generally not suitable for realtime (or perhaps this has changed due to the hardwares we have these days).
But like everyone else mentioned here, for quick fix - Poser, Daz 3d, MakeHuman, Quidam. Poser is probably your best bet since it has a HUGE community with a lot of assets that you can just buy off and customize yourself. But if you want even more control and customizability - you just have to either get an artist, or learn to use one of the 3D programs yourself.
You can’t use DAZ models in a game without a $500 license; that’s on top of the cost of the individual models. That’s not 500 for each model - the 500 is a one-time cost that allows you to use any DAZ Original model you own in as many games as you want - beware! DAZ also has brokered models that you cannot use or require separate license.
Not sure if you can use Poser models in games at all. You would have to check with them before you get too far otherwise you’re setting yourself up for legal problems down the road.
Keep in mind very few of these models, DAZ or Poser are suitable for use in games without first reducing their polygon count significantly and in many cases, makes the models unusable.
Makehuman is free. Daz Studio is free as well, I believe. Daz make money hawking asset packs to you later. What you get with the program is fairly limited.
But, a Daz figure is like 500k to 1 million polygons. In a computer game you have a budget of maybe 3000. I don’t actually know the polygon count for MakeHuman figures, but I think it too exceeds the poly-budget by far. The reason you have a tight budget is because models consume memory, and with one unmodified Daz-figure, you’ll barely have any memory left to do all the other game things. Now, if you put in a dozen or so NPCs or enemies with the same polycount - you’re looking at a slide show game. 
Isn’t it entirely depending on what he wants to make? Like what I mentioned previously - in 2D point-and-click adventure games these programs would be fine because you would just render them out to 2D sprites.
a slight thread ressurrection, but I thought I’d mention turbosquid. There are a Lot of either free or cheap 3D models of all shapes and sizes available, plus assorted texture resources. Good place to grab assets.